1. Field of the Invention
The disclosure is generally related to haptic systems employing force feedback.
2. Description of the Related Art
Touch, or haptic interaction is a fundamental way in which people perceive and effect change in the world around them. Our very understanding of the physics and geometry of the world begins by touching and physically interacting with objects in our environment. The human hand is a versatile organ that is able to press, grasp, squeeze or stroke objects; it can explore object properties such as surface texture, shape and softness; and it can manipulate tools such as a pen or wrench. Moreover, touch interaction differs fundamentally from all other sensory modalities in that it is intrinsically bilateral. We exchange energy between the physical world and ourselves as we push on it and it pushes back. Our ability to paint, sculpt and play musical instruments, among other things depends on physically performing the task and learning from the interactions.
Haptics is a recent enhancement to virtual environments allowing users to “touch” and feel the simulated objects with which they interact. Haptics is the science of touch. The word derives from the Greek haptikos meaning “being able to come into contact with”. The study of haptics emerged from advances in virtual reality. Virtual reality is a form of human-computer interaction (as opposed to keyboard, mouse and monitor) providing a virtual environment that one can explore through direct interaction with our senses. To be able to interact with an environment, there must be feedback. For example, the user should be able to touch a virtual object and feel a response from it. This type of feedback is called haptic feedback.
In human-computer interaction, haptic feedback refers both to tactile and force feedback. Tactile, or touch feedback is the term applied to sensations felt by the skin. Tactile feedback allows users to feel things such as the texture of virtual surfaces, temperature and vibration. Force feedback reproduces directional forces that can result from solid boundaries, the weight of grasped virtual objects, mechanical compliance of an object and inertia.
Tactile feedback, as a component of virtual reality simulations, was pioneered at MIT. In 1990 Patrick used voice coils to provide vibrations at the fingertips of a user wearing a Dextrous Hand Master Exoskeleton. Minsky and her colleagues developed the “Sandpaper” tactile joystick that mapped image texels to vibrations (1990). Commercial tactile feedback interfaces followed, namely the “Touch Master” in 1993, the CyberTouch® glove in 1995, and more recently, the “FEELit Mouse” in 1997.
Scientists have been conducting research on haptics for decades. Goertz at Argonne National Laboratories first used force feedback in a robotic tele-operation system for nuclear environments in 1954. Subsequently the group led by Brooks at the University of North Carolina at Chapel Hill adapted the same electromechanical arm to provide force feedback during virtual molecular docking (1990). Burdea and colleagues at Rutgers University developed a light and portable force feedback glove called the “Rutgers Master” in 1992. Commercial force feedback devices have subsequently appeared, such as the PHANTOM™ arm in 1993, the Impulse Engine in 1995 and the CyberGrasp® glove in 1998.
Haptic devices (or haptic interfaces) are mechanical devices that mediate communication between the user and the computer. Haptic devices allow users to touch, feel and manipulate three-dimensional objects in virtual environments and tele-operated systems. Most common computer interface devices, such as basic mice and joysticks, are input-only devices, meaning that they track a user's physical manipulations but provide no manual feedback. As a result, information flows in only one direction, from the peripheral to the computer. Haptic devices are input-output devices, meaning that they track a user's physical manipulations (input) and provide realistic touch sensations coordinated with on-screen events (output). Examples of haptic devices include consumer peripheral devices equipped with special motors and sensors (e.g., force feedback joysticks and steering wheels) and more sophisticated devices designed for industrial, medical or scientific applications (e.g., PHANTOM™ device).
Haptic interfaces are relatively sophisticated devices. As a user manipulates the end effecter, grip or handle on a haptic device, encoder output is transmitted to an interface controller at very high rates. Here the information is processed to determine the position of the end effecter. The position is then sent to the host computer running a supporting software application. If the supporting software determines that a reaction force is required, the host computer sends feedback forces to the device. Actuators (motors within the device) apply these forces based on mathematical models that simulate the desired sensations. For example, when simulating the feel of a rigid wall with a force feedback joystick, motors within the joystick apply forces that simulate the feel of encountering the wall. As the user moves the joystick to penetrate the wall, the motors apply a force that resists the penetration. The farther the user penetrates the wall, the harder the motors push back to force the joystick back to the wall surface. The end result is a sensation that feels like a physical encounter with an obstacle.
The human sensorial characteristics impose much faster refresh rates for haptic feedback than for visual feedback. Computer graphics has for many years contented itself with low scene refresh rates of 20 to 30 frames/sec. In contrast, tactile sensors in the skin respond best to vibrations that are more than an order-of-magnitude higher than visual rates. This order-of-magnitude difference between haptics and vision bandwidths often requires that the haptic interface incorporate a dedicated controller. Because it is computationally expensive to convert encoder data to end effecter position and translate motor torques into directional forces, a haptic device will often have its own dedicated processor. This removes computation costs associated with haptics and the host computer can dedicate its processing power to application requirements, such as rendering high-level graphics.
General-purpose commercial haptic interfaces used today can be classified as either ground-based devices (force reflecting joysticks and linkage-based devices) or body-based devices (gloves, suits, exoskeletal devices). The most popular design on the market is a linkage-based system, which consists of a robotic arm attached to a grip (usually a pen). A large variety of linkage based haptic devices have been patented (examples include U.S. Pat. Nos. 5,389,865; 5,576,727; 5,577,981; 5,587,937; 5,709,219; 5,828,813; 6,281,651; 6,413,229; 6,417,638). The arm tracks the position of the grip and is capable of exerting a force on the tip of this grip. To meet the haptic demands required to fool one's sense of touch, sophisticated hardware and software are required to determine the proper joint angles and torques necessary to exert a single point of force on the tip of the pen. Not only is it difficult to control force output because of the update demand, the mass of a robotic arm introduces inertial forces that must be accounted for.
An alternative to a linkage-based device is one that is tension-based. Instead of applying force through links, cables are connected a point on a “grip” in order to exert a vector force on that grip. Encoders can be used to determine the lengths of the connecting cables, which in turn can be used to establish position of the cable connection point on the grip. Motors are used to create tension in the cables, which results in an applied force at the grip. There is only one commercial tension based device available, through a Japanese company called Cyverse.
The SPIDAR-G is a 7 degree of freedom haptic device that allows translational, rotational and grip force. This design resulted from Dr. Seahak Kim's PhD work (Dr. Seahak Kim is currently an employee of Mimic Technologies, Inc.). References for much of Dr. Seahak Kim's work on the SPIDAR-G have been listed above.
Predating Dr. Seahak Kim's work on the SPIDAR-G, Japanese Patent No. 2771010 and U.S. Pat. No. 5,305,429 were filed that describe a “3D input device” as titled in the patent. This system consists of a support means, display means and control means. The support means is a cubic frame. Attached to the frame are 4 encoders and magnetic switches capable of preventing string movement over a set of pulleys. The pulleys connect the tip of each encoder to strings that are wound through the pulleys. Each string continues out of the pulley to connect with a weight that generates passive tension in the string. The ON/OFF magnetic switches allow the strings to be clamped on command from the host computer. The strings connect to the user's fingertip, which are connected to the weights through the pulleys. The user moves his or her fingertip to manipulate an “instruction point” in a virtual environment, which is displayed through a monitor. As the user moves his or her fingertip, the length of the four strings change, and a computer calculates a three-dimensional position based on the number of pulses from the encoder, which indicate the change of string length between the pulleys and the user's finger. If the three-dimensional position of the fingertip is found to collide with a virtual object as determined by a controlling host computer, then the ON/OFF magnetic switch is signaled to grasp some or all of the strings so that movement is resisted. Forces are not rendered in a specific direction, but resistance to movement in some or all directions indicates that a user has contacted a virtual object. When the fingertip is moved outside the boundary of a virtual object, the magnetic switch is turned off to release the strings. The user is then able to move his or her finger freely.
The “3 dimensional input device” introduced in U.S. Pat. No. 5,305,429 is not capable of controlling or rendering directional forces. It is therefore not possible to render three-dimensional forces in the manner typically associated with a haptic device. In other words, this device is not capable of simulating haptic effects such as the feel or contact with a virtual three-dimensional surface.
The “3 dimensional input device” cannot render controlled vector forces because of the following reasons, (i) it is impossible to display exact directional force, because the device can only apply drag, or resistance to movement to each string by ON/OFF magnetic switches and cannot impose an exact tension in each string; (ii) there is no accounting for the changing force applied by the weights attached to each string which provide a variable tension as the velocity of the moving weight changes the tension; (iii) there is no accounting for extraneous forces resulting from friction between the frame, pulleys, ON/OFF magnetic switches and the strings; and (iv) there is no initialization sequence described which is required for determining initial string lengths as need for determining string orientations and finger position so that forces can be reflected accurately. In summary this is not a true force feedback device, but instead is only a tracking mechanism with a single force effect (direction nonspecific drag). As an input device, the system also lacks a robust measurement method for determining the length of the strings, which results in substantial fingertip position measurement errors. There is also no means for measuring orientation of the finger (roll, pitch and yaw).
A system that combines virtual reality with exercise is described in U.S. Pat. No. 5,577,981. This system uses sets of three cables with retracting pulleys and encoders to determine the position of points on a head mounted display. Using the lengths of the three cables, the position of the point in space is found. Tracking three points on the helmet (9 cables) allows head tracking of 6 degrees of freedom. Three cables attached to motor and encoders are also used to control the movement of a boom that rotates in one dimension through a vertical slit in a wall. The boom also has a servomotor at its end, about which the boom rotates. It is claimed that the force and direction of force applied by the boom can be controlled via the cables, servo motor and computer software, but no details are provided for how this is accomplished.
Applications have been filed for patents in Japan and the US to support the SPIDAR-G (as discussed earlier). This apparatus was titled “three-dimensional input apparatus” and was filed under patent No. 2001-282448 in Japan, and a U.S. patent application has also been submitted: No. 20010038376. This device also consists of a support means, display means and control means similar to that described in U.S. Pat. No. 5,305,429. However, this device uses “at least seven strings” to accommodate “at least six degrees of freedom” (actually the patent application only explains 7 degrees of freedom with 8 strings, but a claim is made for 6 degrees of freedom with seven strings). The mentioned support means includes a cubic frame, where a motor and encoder are attached to at least seven locations on the frame. The tip of each encoder is connected to a spool on a motor, and this spool winds the string. The encoder and motor are rotated simultaneously. The motors are meant to create tension in the strings in place of the weights used in the earlier patent. Instead of the string attaching to the users finger, the strings attach to a sphere shaped grip.
This grip has a mechanical structure that consists of two poles that are rotated based on the grasp force between the thumb and other fingers of the user. Two strings are connected to each of the four extremities of the two poles. If the user moves the tool to manipulate a virtual object in a virtual environment, as displayed through a monitor, each string length is changed, and the computer is used to calculate a three dimensional position (translation, rotation and grasp information) based on the number of pulses from the encoder, which is an indication of a change in string length. If contact is made with a virtual object, the controlling computer is used to calculate the tensions in the strings that are necessary to render force and torque at the grip.
The system described in Japanese patent 2001-282448 can therefore actively display translation, rotation, and grasp force. As a result, the user is able to “touch” virtual objects through the display of force. However, the above-mentioned “3 Dimensional interface device” requires at least seven strings to determine position and render force, even if rotation and grip forces are not being applied, and, while the design described above is adequate for displaying seven degrees of force feedback, there are severe limitations imposed if less than seven degrees of force feedback are required.